Seismicity in the southwestern Gulf of Mexico: evidence of active back arc deformation

Abstract

The earthquakes of 23 May 2007 (Mw5.6) and 29 October 2009 (Mw 5.7) that occurred in the southern continental margin of the Gulf of Mexico were studied by conducting a full wave inversion of teleseismic P waves. In this region, there is a band of shallow seismicity along the continental margin that extends from the Isthmus of Tehuantepec to the city of Veracruz, Mexico. The focal mechanism of the 2009 event shows reverse faulting at a high angle and is very similar to that of the 1959 Jáltipan earthquake (Mw 6.4) and to the 1973 Veracruz event (Mw 5.3). The focal depths between 22 and 27 km of these three earthquakes are unusually deep for continental events. The location of this band of seismic activity and the focal mechanisms suggest a process of crustal shortening of the southern margin of the Gulf of Mexico. This compressive regime appears to be induced by the subduction of the Cocos plate to the south, in a manner that is reminiscent of the crustal shortening process observed in the Andes. The 2007 earthquake took place north of this region, seaward of the city of Tuxpan. The focal mechanism shows strike slip faulting and the hypocentral depth obtained from the body wave inversion is approximately 7 km. The source mechanism of this earthquake and the depth in the upper crust where it took place suggest a different tectonic process than the one observed near the Isthmus of Tehuantepec.